drm/i915: extract fbc_on_pipe_a_only()

Make the code easier to read.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446664257-32012-3-git-send-email-paulo.r.zanoni@intel.com


Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>